This invention relates in general to digital communication systems, and in particular to an improved partial response digital data transmission system.
Digital communication systems, such as microwave systems, have been developed to provide high quality, reliable communications for digitized voice, data and video signals. Such systems are particularly useful for interlinking computers and other digital sources over long and short-haul transmission media.
Partial response (PR) and quadrature partial response (QPR) transmission techniques have been used extensively in microwave, cable and twisted-wire digital transmission systems. An advantage of the PR and QPR transmission techniques (also known as correlative and/or duobinary transmission techniques), lies in their relatively simple filtering strategy and implementation. Also, to a limited extent, it is possible using prior art PR and QPR techniques, to transmit data at rates which exceed the well known Nyquist transmission rate R.sub.N =2f.sub.N, where f.sub.N =Nyquist frequency.
A detailed description of partial response, or correlative coding techniques, is presented by Dr. A. Lender in Chapter 7 of a reference book by Dr. K. Feher entitled Digital Communications: Microwave Applications, Prentice Hall Inc., 1981, as well as in several patents which have issued on the subject.
U.S. Pat. No. 3,573,622, entitled Partial Response Signalling Sampling for Half Speed Data Transmission, discloses a technique for modifying signalling characteristics of a normal multi-level partial response line signal to provide a two level line signal for facilitating initialization procedures, such as timing recovery and automatic equalizer control. It also provides means for signalling at one-half the Nyquist rate without changing channel filters or the basic clock rate.
U.S. Pat. No. 3,947,767, entitled Multi-level Data Transmission System, discloses a unique differential coding scheme utilized in a partial response system for resolving phase ambiguity difficulties which are introduced by carrier recovery circuitry.
U.S. Pat. No. 4,123,710, entitled Partial Response QAM Modem, discloses a data communication system which combines QAM and partial response techniques. In particular, an optimum combination of multi-level encoding and QAM and partial response techniques are employed to provide digital transmission through a telephone voice channel at a specific data rate, less than or equal to the Nyquist rate.
U.S. Pat. No. 4,439,863, entitled Partial Response System with Simplified Detection, discloses a simple means for demodulating and detecting eight PSK/PRS signals which have a complex output signal set which can not normally be detected in a straight-forward manner. The disclosed technique utilizes two dimensional decision feedback to translate incoming quadrature signals to a new origin and then enables the use of a conventional 8-PSK decoder/detector which detects only the reduced signal set, not the complete output signal set.
None of the prior art U.S. patents relate to transmission above the Nyquist rate. Conventionally, transmission above the Nyquist rate in PR and QPR systems is achieved by simply increasing the signalling rate with the transmission characteristics being kept fixed. It has been found through experimentation, as discussed below, that reasonably good performance can be achieved with above Nyquist rate signalling in the case of 3-level partial signals, however performance deteriorates as the number of signalling levels is increased above three.
Prior art PR and QPR system block diagrams are shown in FIGS. 1A and 1B respectively. For example, with reference to FIG. 1A, a data signal is transmitted at a binary rate of f.sub.b =800 kb/s and is applied to a digital-to-analog converter 1, which in response generates an 8-level output signal at a rate of R.sub.sa =800 kb/s/3=266 ksymbol/s. The output signal is applied to a duobinary encoder 3 which generates a 15-level signal which is band limited to the Nyquist frequency f.sub.N =R.sub.sa /2=133 kHz. Thus, the input binary signal is converted to a 15-level partial response signal having a spectral efficiency of
800 kb/s.apprxeq.133 kHz=6 b/s/Hz or PA1 266 ksymbol/s.apprxeq.133 kHz=2 symbol/s/Hz
at the Nyquist rate. The 15-level partial response signal is then quadrature modulated and transmitted via radio, microwave, etc., through a transmit channel medium subject to extraneous channel noise, and designated by summer 5. The transmitted signal is demodulated by a conventional coherent QPRS demodulator and fed to the receive filter 7 and subsequently reconverted to a digital binary signal via an analog-to-digital converter 9.
With reference to the prior art QPR modem shown in FIG. 1B, a binary data signal is transmitted at a predetermined source rate and applied via serial-to-parallel converter 11 into "in-phase" and "quadrature" signal paths each comprised of a digital-to-analog converter (13 and 15), a duobinary encoder (17 and 19) and a signal multiplier (21 and 23). A local oscillator 25 generates a carrier signal for application to multiplier 21, and via 90.degree. phase shifter 27 to multiplier 23. The orthogonal signals output from multipliers 21 and 23 are summed via signal summer 29 and applied via an optional bandpass filter 31 to the transmit channel 5, for reception by a conventional QPRS demodulator 33.
With reference to FIG. 2A, a prior art duobinary encoder is shown in detail. Data signals are received and transmitted at a rate of R.sub.sa =2f.sub.N (1+a), where "a" represents the amount in percentage by which the transmission rate exceeds the Nyquist rate R.sub.N. The data signals are typically filtered via an optional analog amplitude equalization filter 40 and applied to a cosine filter 42, typically implemented using well known digital circuitry. The signal output from filter 42 is applied to a rectangular transmission filter 44 having a cut-off frequency equal to the Nyquist frequency of f.sub.N.
The combined transfer function H(f) due to filters 42 and 44 is illustrated in FIG. 2B, wherein H(f)=H.sub.1 (f).multidot.H.sub.2 (f).
It has been found that the prior art system of FIG. 2A exhibits approximately 43% speed-tolerance when using 3-level duobinary encoding, and approximately 8% tolerance when using 5-level duobinary encoding. In other words, using a 5-level system based on the prior art duobinary encoder of FIG. 2A, data signals can be transmitted at only 8% above the Nyquist rate before intersymbol interference becomes too great for error free transmission.
A convenient and well known technique for evaluating intersymbol interference is by means of an eye diagram, or eye pattern. Eye diagrams are described for example on page 52 of the aforementioned reference book by K. Feher.
An eye diagram for the 3-level PR (corresponding to a 9-QPR) system transmitting at a Nyquist rate of R.sub.N =2 bits/second/Hertz and utilizing the prior art duobinary encoder discussed with reference to FIG. 2A, is characterized by completely open "eyes" indicating no intersymbol interference, i.e a completely open "vertical" eye diagram.
Eye diagrams for the 3-level PR system transmitting at data rates greater than the Nyquist rate by a=20% and a=40% respectively, when using the encoder of FIG. 2A, were characterized by increasingly degraded eyes, i.e. reduced vertical eye openings. In particular, at a=40% above the Nyquist rate R.sub.N, the spectral efficiency was found to be 2(1+a)=2.8 bits/second/Hertz, and the eye diagram was almost completely closed, indicating a substantial amount of intersymbol interference.
The eye diagram for a prior art 7-level PR (or equivalent 49-QPR system) operating at the Nyquist transmission rate, was characterized by substantially open eyes and a spectral efficiency of 4 bit/second/Hertz while the same system operating at a=20% was characterized by a practically closed eye diagram such that, at the sampling instants it was not possible to clearly distinguish between the 7-levels.
The eye diagram for a prior art 15-level PR (or 225-QPR) system transmitting at 5% higher than the Nyquist rate, was characterized by a spectral efficiency of 6.30 bits/second/Hertz and almost completely closed eyes, while the eye diagram at 7.5% higher than the Nyquist rate was characterized by a completely closed eye.
Therefore, it is apparent that in the case of multi-level systems, the speed tolerance of prior art PR and QPR transmission systems is dramatically reduced in relation to binary or tertiary systems. For example, the 43% speed tolerance exhibited by the aforementioned 3-level duobinary system is reduced to less than 20% for a 7-level or higher system.
The shortcomings of prior art PR and QPR transmission systems can be appreciated with reference to a practical application. For highly spectral efficient applications, such as "data-in-voice" supergroup modem applications, data source transmission rates of up to f.sub.b =l.544 Mb/s are required, which with the inclusion of overhead bits, results in a total rate of approximately f.sub.b =1.6 Mb/s or higher, for transmission in a bandwidth of 256 kHz. With the inclusion of side band filters for establishing CCITT Standard FDM-Multiplex guard bands, the transmission bandwidth is reduced to approximately 240 kHz. Thus, a spectral efficiency of 6.66 b/s/Hz or more would be required for accurate transmission. The theoretical Nyquist rate of a 15.times.15=225 QPRS modem is 6 b/s/Hz. In order to transmit a 1.6 Mb/s data signal by means of a standard 225-QPRS modem, in a supergroup 240 kHz band, a transmission rate which is 11% higher than the Nyquist rate would be required. An eye diagram for such a system according to prior art techniques with a=11%, is characterized by complete eye closure as discussed above, yielding an unusable system.